The present invention relates to an electro-optical distance measurement and a distance measuring device which comprises an electro-optical distance measuring unit. In particular, the present invention relates to an electro-optical distance measuring method, a distance measuring program and a distance measuring device, by which it is possible to measure distances to multiple points at one time.
An electro-optical distance measuring unit has a distance measuring light optical path for projecting a laser beam to an object to be measured and for receiving a reflected light from the object to be measured. In addition to the distance measuring light optical path, the electro-optical distance measuring unit has an internal reference light optical path, and a distance to the object to be measured is measured based on a time difference between an internal reference light received via the internal reference light optical path and a distance measuring light received via the distance measuring light optical path.
In recent years, a pulsed laser beam has been used as the laser beam for distance measurement, and photodetection time difference between the internal reference light and the distance measuring light is calculated based on the deviation of the time of pulsed light. An electro-optical distance measuring unit to be arranged on a distance measuring device is disclosed in JP-A-2004-144681, for instance. Referring to FIG. 12, brief description will be given below on the electro-optical distance measuring unit as disclosed in JP-A-2004-144681.
In FIG. 12, reference numeral 1 denotes an electro-optical distance measuring unit. The electro-optical distance measuring unit 1 comprises an optical system 2 and a measured distance calculating unit 3.
The optical system 2 has a distance measuring light optical path 4 and an internal reference light optical path 5. On the distance measuring light optical path 4, there are provided a laser light source 6, a right-angle reflection mirror 7, an objective lens 8, and a photodetection element 9. The laser light source 6 is a pulsed laser diode (PLD), for instance. From the laser light source 6, a distance measuring light, i.e. a pulsed laser beam, is emitted. The distance measuring light is deflected by the right-angle reflection mirror 7 and is projected from the electro-optical distance measuring unit 1 through the objective lens 8. The distance measuring light thus projected is reflected by an object to be measured 11, i.e. a prism. The reflected distance measuring light is entered via the objective lens 8 and is deflected and is received by the photodetection element 9.
A half-mirror 12 is arranged on the distance measuring light optical path 4. A part of the distance measuring light is reflected as an internal reference light by the half-mirror 12. The internal reference light is deflected by a reflection mirror 13, and the internal reference light optical path 5 is formed. On the internal reference light optical path 5, there are arranged relay lenses 14 and 15. After passing through the relay lens 15, the internal reference light is deflected by a reflection mirror 16. The internal reference light is reflected along the distance measuring light optical path 4 by a half-mirror 17 arranged on the distance measuring light optical path 4 and is received by the photodetection element 9.
An optical path switchover device 18 is positioned to stretch over an outgoing portion of the distance measuring light optical path 4 and the internal reference light optical path 5, and a light amount adjusting device 19 is arranged to stretch over a returning portion of the distance measuring light optical path 4 and the internal reference light optical path 5.
The optical path switchover device 18 comprises a rotary light shielding plate 21. When one of the distance measuring light optical path 4 and the internal reference light optical path 5 passes through the light shielding plate 21, the other of the optical paths is shut off by the light shielding plate 21. The rotary light shielding plate 21 is designed as to be rotated by an actuator 22 such as a motor. The light amount adjusting device 19 comprises a light amount attenuating filter 23. The light amount attenuating filter 23 is rotated by an actuator 24 such as a motor, and the light amount is adjusted so that light intensity of the distance measuring light entering the photodetection element 9 will be equal to or approximately equal light intensity of the internal reference light.
After being received by the photodetection element 9, the distance measuring light and the internal reference light are sent to the measured distance calculation unit 3 as photodetection signals.
As the result of switchover of the optical paths by the optical path switchover device 18, the distance measuring light and the internal reference light divided in time series enter alternately the photodetection element 9. From the photodetection element 9, a photodetection signal of the distance measuring light and a photodetection signal of the internal reference light are sent alternately to the measured distance calculating unit 3. At the measured distance calculating unit 3, the pulses of the distance measuring light are compared with the pulses of the internal reference light and a deviation is calculated. Based on the deviation thus obtained, a distance to the object to be measured 11 is calculated.
In the electro-optical distance measuring unit 1 of the conventional type distance measuring device as describe above, optical paths of the distance measuring light and the internal reference light are mechanically switched over by the rotary light shielding plate 21. As a result, there has been such problems that there is limitation in responsiveness and so on, and it is difficult to switch over the optical paths at high speed and to measure a distance at high speed. The distance measuring device as described in JP-A-2004-144681 is to measure one object to be measured at one time measurement, and it is not possible to measure distances to the objects to be measured at a multiple of points at the same time.
A distance measuring device for electrically switching over the photodetection of the distance measuring light and the internal reference light is disclosed in JP-A-2006-105802.
A distance measuring device, which can perform measurement on a plurality of objects to be measured at one time, is disclosed in JP-A-5-232228.
According to the distance measuring device described in JP-A-5-232228, a pulsed light is emitted, and a time (distance measuring time) from the moment of the pulsed light emission to the moment when the reflected pulsed light from the object to be measured is received is determined, and a distance to the object to be measured is measured. This distance measuring device comprises a semiconductor laser to emit the pulsed light, a photodetection element to issue a photodetection signal by receiving the pulsed light reflected from the object to be measured, time measuring circuits as many as the number of the objects to be measured and flip-flop circuits to match the time measuring circuits, and a distance value calculating device to calculate a distance to the object to be measured based on the distance measuring time. The flip-flop circuit sends a photodetection signal to the matching time measuring circuit upon receipt of the signal from the photodetection element and attains non-deliverable condition to the subsequent photodetection signal.
Therefore, a plurality of the flip-flop circuits send the photodetection signals one after another to the matching time measuring circuits and further attain the non-deliverable conditions one after another. As a result, the plurality of the time measuring circuits can individually measure the distance measuring time for each of the objects to be measured, and the distance value calculating device can measure the distance to the object to be measured based on the distance measuring time.
According to the distance measuring device as disclosed in JP-A-5-232228, it is possible to measure distances to a plurality of objects to be measured at one time.
However, according to the distance measuring device described in JP-A-5-232228, the time measuring circuit and the flip-flop circuit are required for each object to be measured. In order to perform the measurement many times to multiple points and to improve the accuracy, a great number of the time measuring circuits and the flip-flop circuits are required, and complicated circuit arrangement must be designed, and this means the increase of the costs. Also, due to the electrical switchover operation by the flip-flop circuit, response delay occurs on the circuit. As a result, there is a range where a signal cannot be measured after receiving the last receiving signal. This range is turned to a dead zone, and the problem arises in that measurement cannot be made on the objects to be measured, which are closer to each other at a distance to match the response delay time on the circuit.